1. Field of the Invention
The present invention relates to a recording sheet, more precisely, to a novel recording sheet having a color developer layer capable of reacting with a color former to form a developed color image.
2. Description of the Prior Art
Recording sheets are, in general, well known where a developed color image is formed by a so-called color forming reaction of a substantially colorless electron donating organic compound capable of forming a color (hereinafter referred to as a "color former") on contact with an electron accepting solid acid (hereinafter referred to as a "color developer"). Examples of color developers are clay substances such as terra alba, activated clay, attapulgite, zeolite, bentonite and kaolin; organic acids such as succinic acid, tannic acid, gallic acid, alkyl substituted phenols and bisphenol A; metal salts of aromatic carboxylic acids such as zinc salts and aluminum salts of alkyl substituted salicylic acids; and acid polymers such as p-phenylphenol/formaldehyde resins. The term "color former" as used hereinafter means an electron donating or proton (for example, from an acid) accepting material capable of forming a color image, and the term "color developer" as used hereinafter means an electron accepting or proton donating material.
Recording sheets where these phenomena are specifically used include pressure-sensitive copying papers (for example, as disclosed in U.S. Pat. Nos. 2,505,470, 2,505,489, 2,550,471, 2,548,366, 2,712,507, 2,730,456, 2,730,457 and 3,418,250) and heat-sensitive recording papers (for example, as disclosed in Japanese Patent Publication No. 4160/68 and U.S. Pat. No. 2,939,009). Heat-sensitive recording papers are obtained by coating a color former and a color developer on a support together with a heat fusible substance such as acetanilide. A heat fusible substance means a substance capable of melting on heating to dissolve a color former. Color developers having a low melting point themselves may be used without a heat fusible substance.
Another printing method is known where an ink containing a color former is supplied to a color developer coated sheet through a medium such as stencil to obtain a developed color image (for example, as disclosed in German Patent Application (OLS) No. 1,939,624).
The recording using above-described phenomena requires a physical stimulus such as application of a pressure with a ball-point pen or a typewriter or heating with a heating element or an electric current, etc.
The most typical embodiment of a recording sheet is a pressure-sensitive copying paper, which is prepared by dissolving a color former in a solvent such as an alkylated naphthalene, alkylated diphenylmethane, alkylated diphenyl, terphenyl or chlorinated paraffin, dispersing the resulting solution in a binder or encapsulating the resulting solution in microcapsules, and then coating such on a support such as paper, a plastic sheet or a resin coated paper. On the other hand, where a color developer is dissolved in a solvent such as an organic solvent, the color developer solution in situ penetrates into a support, or after being dissolved or dispersed in a medium together with a binder, this is coated on a support. In general, color former and color developer are coated on the same surface or opposite surfaces of a support or on different supports.
Some problems which occur with these conventional color developer coated sheets need to be improved. More precisely, where clay substances such as activated clay are used as a color developer, the developed color image (for example, Crystal Violet formed from crystal violet lactone as a color former) fades away on contact with polar solvents such as water or glycols, which is a disadvantage.
In addition, where phenol resins such as p-phenylphenolformaldehyde resin are used as a color developer, a yellowing of the surface to be developed easily occurs due to the effects of light, which is also a defect. Moreover, aldehydes such as formaldehyde are used in the synthesis of phenol resins and the use of highly toxic formaldehyde results in various difficulties occurring during the synthesis steps and is not preferred. Where phenol compounds such as alkyl substituted phenols and bisphenol A are used as a color developer, these compounds tend to sublime or diffuse in a support with the lapse of time, and as a result, the color developability gradually decreases, which is a disadvantage. In addition, the use of alkyl substituted phenols and bisphenol A is not preferred because the initial color developability is low. These defects greatly affect the product value and it is desired to improve these problems.
It was previously found that dimers of compounds of the following formula (I) (hereinafter referred to as alkenyl phenols) were effective as a color developer for recording sheets: ##STR2## wherein R.sub.1, R.sub.2 and R.sub.3 each represents a hydrogen atom, an alkyl group, an aryl group or an aralkyl group, and n is an integer of 1 to 4. The use of the above-described alkenyl phenol dimers results in a substantial improvement in various properties of the color developer. However, the color developability of the coated layer and the film surface strength are still not sufficient, and improvements must further be made.